Absolute Threshold PSY 3215 – Perception – Appalachian State University

Method of Limits  A range of stimuli are selected, from below threshold to above threshold.  The stimuli are ordered and presented in a sequence that corresponds to either an ascending or descending series of the “amount” of the physical dimension under investigation.  Forced Choice: The observer is to judge only whether the stimulus was detected (Yes) or not detected (No).  The trial is ended when there is a transition.

Method of Limits  The observer will need to make many judgments over several repetitions of the ascending and descending series.  The psychophysical function is created in the following fashion:  X-axis contains the information about the physical dimension. Stimuli are ordered from less to more. Interval size reflects size of increment from stimulus to stimulus.  Y-axis contains the information about the observer’s judgment of the experience. Responses are reported as Percent(Yes). Scale goes from 0.00 (0%) to 1.0 (100 %)

Let’s try an example  The following slide contains a series of music icons.  Clicking on an icon will cause a 2-second tone to be played.  You should record Y if you hear the tone and N if you do not hear the tone.  The number on the icon indicates the “amount” of volume.  Each increment in number is an equal increment in volume.

Let’s try an example  First, the speaker volume will be adjusted so that the range of stimuli goes from inaudible to audible.  Second, you will make judgments with ascending and descending series.  Third, we will compute the psychophysical function and discuss the location of the absolute threshold.

The Stimuli

Method of Constant Stimuli  A range of stimuli are selected, from below threshold to above threshold.  The stimuli are ordered and presented in a randomized series of the “amount” of the physical dimension under investigation.  Forced Choice: The observer is to judge only whether the stimulus was detected (Yes) or not detected (No).

Method of Constant Stimuli

Method of Adjustment  Observer makes adjustment to stimulus source until threshold is achieved.  A production method. (more Behavioral?)  Method works very quickly.  Greater variability hence less accuracy.

Staircase Method  Presents and notes response (“Yes” or “No”)  If “No” then increase stimulus intensity on next trial.  If “Yes” then decrease stimulus intensity on next trial.  Direction of change is reversed when answer changes.